These investigations are devoted to the development of non-invasive methods of accessing tissue structure and function. Two general techniques are being developed: nuclear magnetic resonance (NMR) and optical spectroscopy/imaging. Over the last year we have made the following developments in NMR: 1) The water relaxation rates the head and musculo- skeletal system has been determined as well as their relevance to MRI tissue contrast at 4T. 2) A negative correlation between skeletal muscle free intracellular Mg and plasma values was established using 31P NMR. These suggests that the intracellular pool may serve as a Mg buffer pool for the plasma. 3) A radiation shield was developed and characterized for use on surface coils at 4.0 T in humans. 4) Experiments on ordered lipid models showed the transverse diffusional coefficient of water between tightly packed bylayers is only restricted by a factor of 3 and its activation energy is similar to free water. 5) Vascular gas bubbles and met-hemoglobin were evaluated as MRI contrast agents for non-invasive determinations of vascular pressure and flow respectively. In optical spectroscopy/imaging: 1) A laser rapid scanning spectrophotometer has provided the first in vivo measures of mitochondrial NADH and oxygenation levels in the heart. 2) Laser pumped "super polarized" Xe for in vivo tracer NXR studies has been created. Magnetic relaxation rates of Xe in biological tissues are being determined for tracer studies.